1. Field of the Invention
The present invention relates to a dielectric resonator apparatus, and more particularly, to a dielectric resonator apparatus comprising a plurality of dielectric coaxial resonators, which is generally used as a dielectric filter.
2. Description of the Prior Art
Conventionally, there has been widely used as a dielectric band-pass filter for microwave band frequencies, for example, a dielectric resonator apparatus comprising a plurality of dielectric coaxial resonators, each of which is formed by forming a resonator hole in an inner portion of a dielectric block, forming an inner electric conductor in the resonator hole and forming an outer electric conductor on an outer surface of the dielectric block. In the specification, an electric conductor is referred to as a conductor hereinafter.
FIG. 9 is a perspective view showing an appearance of a first conventional dielectric resonator apparatus. In the first conventional dielectric resonator apparatus, a pair of input and output electrodes is formed on the outer surface of the dielectric block in the above-mentioned dielectric resonator apparatus, and the longitudinal length of the resonator holes or of the axis of each resonator is set to half the guide-wavelength .lambda.g/2, resulting in a plurality of half-wavelength dielectric coaxial resonators.
Referring to FIG. 9, two circular cylindrical resonator holes 2a and 2b are formed in parallel to each other so as to penetrate a rectangular-parallelepiped-shaped dielectric block 1 between first and second surfaces S1 and S2 of the dielectric block 1, and an inner conductor is formed on an inner peripheral surface of each of the resonator holes 2a and 2b. Further, an outer conductor 6 is formed on the first and second surfaces S1 and S2 and the side surfaces S3, S4, S5 and S6 of the dielectric block 1. Furthermore, a pair of input and output electrodes 7a and 7b is formed on the side surface S3 of the dielectric block 1 so as to be electrically insulated from the outer conductor 6. Thus, there can be constituted the first conventional dielectric resonator apparatus comprising the two half-wavelength dielectric coaxial resonators.
FIG. 10 is a perspective view showing an appearance of a second conventional dielectric resonator apparatus having first, second and third cylindrical resonator holes 2a, 2b and 2c, an outer conductor 6, and input and output electrodes similar to those in FIG. 9. As shown in FIG. 10, in the case of the dielectric resonator apparatus of three or more stages, it is necessary to change the lengths of the axes of the dielectric coaxial resonators, and therefore, it is necessary to form steps in the dielectric block 1.
Since both of the opening surfaces S1 and S2 of the resonator holes 2a and 2b in FIG. 9 are short-circuit surfaces, no leakage of the electromagnetic field occurs there. However, a current concentration is caused at a short-circuit end of the inner conductor, namely, at a boundary between the inner conductor and the outer conductor 6, resulting in a deterioration in the unloaded Q (Q.sub.0) of each of the dielectric coaxial resonators.
Further, as shown in FIG. 10, in the dielectric resonator apparatus of three or more stages, it is necessary to process the dielectric block 1, resulting in an increase in the manufacturing cost.